The present invention relates to light-sensitive compounds and compositions suitable for use in the graphic arts. More particularly, the present invention relates to the use of the condensation products of organic isocyanates and polymeric materials, carrying functional groups which are capable of reacting with the organic isocyanates, in the manufacture of printing plates and photoresists.
In the manufacture of positive-working printing plates and photoresists, derivatives diazides, as disclosed in U.S. Pat. Nos. 3,046,110-3,046,119, 3,046,121-3,046,124, 3,106,465, 3,148,983, 3,180,733, and 3,188,210, are widely used as light-sensitive materials. These naphthoquinone diazides are generally soluble in certain organic solvents but are not soluble in water, weak acids and weak alkalies. Coating solutions can be prepared utilizing one or more of these naphthoquinone diazide light-sensitive compositions in organic solvents and the solution then coated on a suitable substrate such as aluminum, zinc, copper, plastics, paper, etc. The choice of the substrate depends on the intended use of the structure. When a substrate, coated with one of these light-sensitive compositions, is exposed to light through an imaged transparency, naphthoquinone diazides, in the exposed areas, are believed to be decomposed to the indene carboxylic acids, which are soluble in a weak alkaline solution. Therefore, through the action of the light, a solubility differential between the exposed and unexposed areas results. The image can then be developed out by treatment with a proper weak alkaline developer solution and removal of the solubilized non-image areas.
The compounds described in the above-mentioned U.S. patents, many of which have been reported elsewhere, are low molecular weight esters and acid amides of quinone of naphthoquinone diazide. If such a compound is used individually, for instance, in the manufacture of lithographic printing plates, it is deposited in crystalline form, which results in lowering the mechanical strength of the image obtained and making long press runs difficult to attain. Accordingly, a suitable polymeric resinous material is usually used as a carrier for the light-sensitive compound to prevent it from cyrstallizing and to compensate for any weakening of the mechanical strength. Suitable polymeric materials, which are employed for this purpose, are alkali-soluble resins such as shellac, styrene-maleic anhydride copolymers, hydrophobic thermoplastic polyurethanes (U.S. Pat. No. 3,660,097) and, especially, low molecular weight condensation products of phenol and formaldehyde, the so-called `novolaks` (U.S. Pat. Nos. 3,148,983 and 3,188,210 and many others). However, when an alkali-soluble resin, such as the novolak available under the tradename of "Alnovol PN 430", manufactured by Chemische Werk Albert, Wiesbaden-Biebrich, Germany, is used as recommended in the prior art, the unexposed areas of the sensitized coating do not possess the requisite adhesion to the substrate and tend to separate therefrom. Such a tendency becomes particularly manifest during the development operation, i.e., those instances wherein the plate is subjected to continued and prolonged immersion in the developer solution. Therefore, the image contrast and edge sharpness as well as press performance deteriorate. In cases where good plate performance is required, it is necessary to heat-treat the developed plates at an elevated temperature so that a more durable printing surface can be obtained. However, such treatment usually yields a surface with many pinholes and spots. Therefore, the commercial value of such a system is greatly reduced. Conversely, when a hydrophobic polymeric material, such as the "Estane" thermoplastic polyurethanes sold by B. F. Goodrich Company, is used as a carrier (U.S. Pat. No. 3,660,097) for the light-sensitive compounds, considerable difficulty is encountered in removing the exposed area of the sensitized coating, since the aqueous developer solution is uniformly repelled from the surface. Therefore, a preponderance of light-sensitive compounds would have to be incorporated in the coating compositions so that the sensitized layer will be more permeable to the alkaline developer. Use of a high percentage of sensitizer, in this manner, yields coatings which require long exposure time and are slow to develop, besides being unduly costly. In many instances, it is necessary to add a small amount of organic solvents to the alkaline developer to speed up the removal of the hydrophobic polymeric layer. In such cases, the resistance of the image areas to the alkaline developer is greatly reduced.
In accordance with the present invention, these drawbacks of the prior art can be overcome, or at least considerably minimized, by using polymeric materials which have been especially chemically modified for photographic reproduction applications.
One object of this invention is to provide such novel chemically-modified polymeric materials.
An additional object is to provide light-sensitive compositions containing isocyanate-modified polymeric materials.
A further object is to provide light-sensitive compositions, based on isocyanate-modified polymeric materials, and systems using these compositions to prepare lithographic printing plates, photoresists, and similar photomechanical images.
A still further object is to provide light-sensitive elements with improved image adhesion and developer resistance.
Additional objects will be apparent to those skilled in the art from the following detailed description of the present invention.